1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling access point name (APN) based congestion control and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple user equipments (UEs), and communicates with a core network including a mobility management entity (MME), serving gateway, etc for Non Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
An access point name (APN) is used in the LTE system and the LTE-A system for establishing packet data connections, and is managed by the MME. Further, the APN is used for identifying a packet data network (PDN) or a service type (e.g. multimedia messaging service (MMS)), wherein the PDN or the service type is carried on the packet data connections.
In prior arts, if an APN is in congestion, i.e., overloaded, the MME rejects an attach request, a tracking area update, a routing area update or a service request with the APN transmitted by a UE, and the MME replies the UE with a Mobility Management back-off timer. On the other hand, when the MME rejects a PDN connectivity request, a PDN disconnectivity request, an evolved packet system (EPS) activation request or an EPS deactivation request with the APN transmitted by the UE, the MME replies the UE with a Session Management back-off timer. In either case, the UE is not allowed to transmit the same request to the MME, before the corresponding back-timer expires. In other words, the UE cannot transmit the same request for the congested APN to the MME again as long as the corresponding back-off timer is running. However, the UE is allowed to initiate a Mobility Management procedure or a Session Management procedure with the APN for a high priority service or an emergency service, even when the Mobility Management back-off timer or the Session Management back-off timer is running. In this situation, since the APN is still in the congestion and has no resource for providing the high priority service or the emergency service, the MME rejects the high priority service or the emergency service, and a delay to access the high priority service or the emergency service is caused. Therefore, disaster may happen due to the delay of the high priority service or the emergency service.
Besides, when a back-off timer expires, the UE can transmit a request to the MME again. However, it is possible that the APN is still in the congestion and has no resource for accepting the request, and the MME needs to reject the request again. If the APN is in the congestion for a long time, the UE may transmit the request to the MME for a large number of times, and correspondingly, the MME rejects the request for the large number of times until the congestion is resolved, i.e., resource for the request is available. The large number of retransmissions and rejections worsen the congestion of the APN and increases load of the MME. In this situation, not only the delay of request is increased, delays of other requests transmitted by other UEs are also increased.
Therefore, how to handle the high priority service and the emergency service immediately is a question to be discussed and addressed. Further, it is important to mitigate the congestion of the APN such that the MME can accept the request as soon as possible. Therefore, a mechanism for mitigating the congestion of the APN is needed.